Compensation of parasitic or stray magnetic fields, especially aboard an aircraft

ABSTRACT

A magnetometer head generates a e.m.f. f1 at a frequency phi 1 proportional to (Ho+ Delta H1), value of the total magnetic field intensity at a first point aboard a plane or similar object; a frequency meter derives from frequency f1 a voltage e1 proportional to phi 1; at a second point (located aboard said plane or object), where the magnetic field intensity is (Ho+ Delta H2), a nuclear filter is fed by e.m.f. f1 and delivers an e.m.f. f2 out of phase with f1 by d phi proportional to ( Delta H2- Delta H1); a phase-meter is fed by f1 and f2 and delivers a voltage e2 proportional to d phi ; an amplifier having an adjustable gain amplifies the output of said phase-meter and delivers a voltage e3; a subtraction unit receives voltages e1 and e3 and delivers a voltage e4 proportional to (e1-e3), said voltage e4 being a measure of the external magnetic field Ho to be measured as e1 is proportional to (Ho+ Delta H1) and e3 is proportional (with the same proportionality factor) to Delta H1 if Delta x x( cube root Rho -1) and k2. phi /k1 x/3 WITH Delta H1 parasitic magnetic field in the first point, Delta H2 parasitic magnetic field in the second point, Delta X DISTANCE BETWEEN THE FIRST AND THE SECOND POINT, X DISTANCE OF THE FIRST POINT FROM THE BARYCENTER OF THE PARASITIC MAGNETIC SOURCES, K1 THE PROPORTIONALITY FACTOR BETWEEN F1 AND (Ho+ Delta H1), k2 the proportionality factor between f2 and ( Delta H2- Delta H1), phi (function of x) adjustable gain of said amplifier, Rho RATIO BETWEEN Delta H2 and Delta H1, i.e. Rho Delta H2/ Delta H1 In a modification, at the first point a magnetometerhead, identical to the head located in the first point, is substituted to said filter, and a differential frequency-meter fed by the outputs of both heads is substituted to said phase-meter.

nited Salvi fiatent 1 cor/rmsrrranrsr rAaASrTic o [72] lnventor: AntoineSalvi, Fontaine, France [73] Assignee: Commissariat a LEnergie Atomique,

Paris, France [22] Filed: Oct. 115, 1969 [21] Appl. No.: 866,501

[30] Foreign Application Priority Data Oct. 17, I968 France.......l70,274

[52] US. Cl ..324/0.5, 324/8, 324/43 [51] Int. Cl ..Glr 33/08 [58] Fieldof Search ..324/0.5, 43, 4, 8

[56] References Cited UNITED STATES PATENTS 2,802,983 8/1957 Tolles..324/43 2,891,216 6/1959 Linden... .....324/43 3,441,841 4/1969 Salvi..324/0.5 X

Primary Examiner-Michael J. Lynch Attorney-William D. Stokes ABSTRACTFeb. 1. 1197.2

field intensity at a first point aboard a plane or similar object; afrequency-meter derives from frequency f. a voltage e.

proportional to an: at a second point (located aboard said plane orobject), where the magnetic field intensity is (H -rAl-lz), a nuclearfilter is fed by e.m.f.f and delivers an e.m.f. f out of phase with f byd p proportional to (AH: AH1); a phase-meter is fed by f and f; anddelivers a .voltage e proportional to drp; an amplifier having anadjustable gain amplifies the output of said phase-meter and delivers avoltage e;,; a subtraction unit receives voltages e, and e and deliversa voltage 2 proportional to (e -e3), said voltage 2 being a measure ofthe external magnetic field H to be measured as e, is proportional to (HAH and a is proportional (with the same proportionality factor) to withAH parasitic magnetic field in the first point, AH parasitic magneticfield in the second point,

Ax distance between the first and the second point,

x distance of the first point f m the barycenter of the ln amodification, at the first point a magnetomete 7 identical to the headlocated in the first point, is substituted to said filter, and adifferential frequency-meter fed by the outputs of both heads issubstituted to said phase-meter.

571 A magnetometer head generates a e.m.f. f, at a frequency 15claimssnmwmg Figures piroportional to (110+ AH value of the totalmagnetic f 1 m NUCLEAR MAGNE T ONE 751? FILTER HEAD fl w/r Jf fA/[ 77L62 m: 10 J 5 3 an ,1 dzrxmx 2* 6 J J f AMPLIFIE 2 AMPUHE mace--11 5/rnsausn/cr-Msrm METER J5 e 2] Amusmaw- 1 Z9 Z6 GAIN anrunew J7 294 e E6.SUBTRACTION 4 557 OF ]6 FILTERS L 1. q 5' E0 5' REwRD PATENIEMEB WeMAGNETOMETEH HEAD/ fiWGULAR FREQUENCY 4 3 4 6 I X w y X m vw/m 1 m Tl AW 2 mz .TV m\ I m m E m N M noon 3 ANGULAR FREQUENCY INVENTOR ANTOINEFONTAINE SALVI ATTORNEY COMPENSATION OF PARASllTlC R STRAY MAGNETlCFIELDS, ESPECIALLY ABOARD AN AIRCRAFT The present invention relates tomethods and apparatus for the compensation of the parasitic magneticfields and more particularly, though not exclusively, for thecompensation of such fields aboard an aircraft in order to permit a veryaccurate measurement of the intensity of the earth s magnetic field andits variations, using a magnetometer carried aboard the aircraft.

It is primarily the object of the invention to improve said methods andapparatus in respect both to the accuracy of the compensation and to thesuppression of perturbations acting upon the other instruments carriedon the aircraft or other compensated carrier.

In the US. Pat. No. 3,441,841, issued Apr. 29, 1969 and corresponding toCanadian Pat. No. 809,314 issued Mar. 25, 1969 and to French Pat. No.1,485,557 issued May 16, 1967, is described:

a method for the compensation of the parasitic or stray magnetic fields,notably aboard an aircraft carrying a magnetometer, characterized by thesteps of: determining the dif ference in total magnetic field at twodifferent points at which the intensity of the external magnetic fieldis substantially the same but the intensities of the parasitic magneticfield are different; generating a control quantity, more particularly acurrent intensity, substantially proportional to said difference andhence to the parasitic magnetic field; and producing, under the controlof said quantity, a compensating magnetic field directed oppositely tothe parasitic magnetic field and of intensity substantially proportionalto said quantity whereby to cancel out said difference;

an apparatus for performing the above method, characterized by the factthat it comprises, in combination, means for generating an electriccurrent of intensity substantially proportional to the difference in themagnetic fields at points at which the intensity of the externalmagnetic field is substantially the same but the intensities of theparasitic magnetic field are different, coils of conductive wire andmeans for feeding the same with said current, the disposition of saidcoils and the proportionality factor between said current and saiddifference being such that said coils generate a compensating magneticfield which cancels out said difference.

This prior art apparatus consequently included coils which generated acompensating magnetic field capable of affecting the other flightinstruments, making it in turn necessary to take certain precautions.

1n the Tolles US. Pat. No. 2,715,198, granted on Aug. 9, 1955, themagnetic perturbations set up by the eddy currents in an aircraft werecompensated for by generating, on the basis of the indications of amagnetometer of specific type, a differentiator and a coil, a correctivemagnetic field proportional and opposite to the eddy currents.

Further, in his US. Pat. No. 2,891,216, granted on June 16, 1959, FrankX. Linder described the compensation provided for a magnetic detectionsystem aboard an aircraft comprising a source of perturbation, thecompensated system including a first magnetometer placed at adeterminate distance from the source of perturbation, a secondmagnetometer placed at a greater distance from said source than saidpredeterminate distance, these two magnetometers being electricallyconnected in opposition, and means such as an attenuator being providedto nullify the effects of the perturbations in the two magnetometers.

In point of fact, Linders first and second magnetometers are located attwo points in the aircraft where the total magnetic field is H+kh, andH+h, respectively, where His the external magnetic field to be measured,h the parasitic magnetic field at the point where the secondmagnetometer is positioned, and k a constant greater than unity. Theoutput from the first magnetometer is attenuated in the ratio k, and thedifference between the output from the second magnetometer and theattenuated output from the first magnetometer is determined, which givesa signal proportional to The above system has the disadvantage of notoffering a high degree of accuracy for the following reasons:

if his taken to have a value greatly in excess of unity and morespecifically equal to 2, as indicated by way of example in the patent,the two magnetometers will be relatively distant from each other, for they must be positioned at distances proportional to l and 1X1 from thesource of magnetic perturbation, since the magnetic effect variesinversely as the cube of the distance (as indicated in US. Pat. No.2,891,216, column 1, lines 64 through 66); the two magnetometers willconsequently be too remote to be subjected to homothetic stray magneticfields, so that'compensation cannot be rigorously effective on allaircraft headings.

If k is taken to have a low value very close to unity, such as k=20/l9,then the two magnetometers will be sufficiently Jase a subjected tosubstantially homothetic perturbing magnetic fields, but on the otherhand the precision of the system is reduced since it will measure only asmall fraction of the field H to be determined, for, with k==20/ l 9,the measured magnetic field, or (ll/k) H, is equal to (1-19/20)H=H/20;in other words the precision is divided by a factor of 20.

This makes it necessary to adopt a compromise, taking a value for k ofapproximately 10/9, whereby the measured magnitude becomes H/lO, givingan order of precision diminished tenfold.

The present invention has for its object to mitigate the above-mentioneddrawbacks by providing a degree of accura cy at least equal to that ofthe aforecited French Pat. No. 1,485,557 in conjunction with anelectronic compensation that does not create a parasitic magnetic fieldin the area in which the customary aircraft instruments are located.

The invention accordingly consists principally,

in respect of the method, in generating a first voltage proportional,with a first proportionality factor, to the intensity of the totalmagnetic field at a first point located at a first distance from thebarycenter of the perturbing magnetic fields, in generating a secondvoltage proportional, with a secondiproportionality factor, to thedifference between the intensities of the total magnetic field at saidfirst point and at a second point located at a second distance from saidbarycenter, in multiplying said second voltage by a factor dependentupon said first distance, in generating a third voltage proportional,with said first proportionality factor, to the intensity of theperturbing magnetic field at said first point, in subtracting said thirdvoltage from said first voltage whereby to obtain a fourth voltage, andin determining the amplitude of this fourth voltage, which isproportional to the intensity of the external magnetic field to bemeasured at said first point less the influence of the perturbingmagnetic fields, the first distance x, the distance Ax between saidfirst and second points, the first proportionality factor K,, the secondproportionality factor k said factor 111 and the ratio p between theintensities of the perturbing magnetic fields at said second point andsaid first point respectively, being related by the formulas:

in respect ofthe appaTatus, in causing the same to comprise, incombination: a first magnetometer head located at a first point at;afirst distance from the barycenter of the perturbing magnetic fields andcapable of generating at its output an e.m.f. of frequency proportionalto the intensity of the total magnetic field at that first point; afrequency-meter connected to the output of said first magnetometer headwhereby to receive said e.m.f. thereon and deduce a first voltageproportional to said frequency and hence to said intensity of the totalmagnetic field; a second magnetometer head located at a second point ata second distance from said barycenter and forming with said first heada magnetic gradient-meter adapted to deliver a second voltageproportional to the difference between the intensities of the totalmagnetic field at said second and first points respectively; anadjustable-gain amplifier having its input connected to the output ofsaid gradient-meter whereby to receive said second voltage and deducetherefrom a third voltage; a subtraction unit having two inputsconnected respectively to said frequency-meter output whereby to receivesaid first voltage and to said amplifier output whereby to receive saidthird voltage, and an output delivering a fourth voltage proportional tothe difference between said first voltage applied to its first input andsaid third voltage applied to its second input; and means for measuringsaid fourth voltage which is proportional to the intensity of theexternal magnetic field to be measured at said first point, assumingproper adjustment of the gain of said amplifier and determine positionsof said first and second magnetometer heads, substantially on thefore-aft axis of the aircraft or other carrier vehicle whereverpossible.

In the preferred forms of embodiment for compensating for the magneticperturbations set up by eddy currents in an aircraft or other carriervehicle, recourse is had to the generation, notably by means of at leastone coil feedable with a current of adjustable amplitude, in the regionof said second point, of a corrective magnetic field proportional to theperturbing field due to the eddy currents, the amplitude of thiscorrective field being adjusted to cause the barycenter of the magneticperturbations produced by the eddy currents to coincide with thebarycenter of the ferromagnetic masses.

Preferably, said current of adjustable amplitude is supplied by at leastone generating coil fixed to the aircraft or other carrier subjected tothe eddy current generating magnetic fields.

The description which follows with reference to the accompanyingnonlimitative exemplary drawings will give a clear understanding of howthe invention can be carried into practice.

In the drawings:

FIG. 1 is a diagrammatic illustration of a gradient-meter used toaccomplish a preliminary stage in the process of compensating themagnetometer, to wit determining the gradient of the perturbing magneticfields and the position of the barycenter thereof;

FIG. 2 portrays the functional characteristics of a nuclear filtercomprising the gradient-meter of FIG. 1;

FIG. 3 schematically illustrates the structure and operation of aphase-meter likewise comprising the gradient-meter of FIG. 1;

FIG. 4 is a diagrammatic showing of the circuitry comprising thegradient-meter of FIG. 1 and utilized to determine the position of thebarycenter of the perturbing fields;

FIG. 5 is a block diagram illustrating a first embodiment of acompensated magnetometer according to this invention;

FIG. 6 schematically represents, in accordance with the second teachingof the invention, the means for compensating for the magneticperturbations set up by the eddy currents;

FIG. 7 explains the effect produced by the means illustrated in FIG. 6;and

FIG. 8 is a block diagram of a second form of embodiment of acompensated magnetometer according to this invention.

Before describing in detail two preferred embodiments of a compensatedmagnetometer according to the invention, reference will first be made toFIGS. 1 to 3 for discussion of a preliminary stage the object of whichis to study the barycenter of the perturbing fields due to theferromagnetic masses and to determine the position thereof.

Use is accordingly made of a magnetic gradient-meter of the kinddescribed in the applicants French Pat. No. 1,485,556 filed on Feb. 4,1966, namely one having two probes or measuring heads positioned at somedistance (approximately 1.50 meters for instance) from each other, sucha gradient-meter determining the difference (or gradient) between theintensities of the total magnetic field to which the two heads aresubjected.

Determination of the barycenter of the perturbing magnetic fields isbased on the fact that the variations in the external magnetic field(produced by the region overflown by the aircraft) result in a zeromagnetic gradient between the two heads, whereas conversely all themagnetic perturbations caused by the aircraft produce a nonzero magneticgradient.

If the barycenter of the ferromagnetic masses associated with theperturbing aircraft fields should shift while the aircraft is in flight,within a space the dimensions of which are small compared with thedistance between the heads of the magnetic gradient-meter, then thevariations in the magnetic gradient determined by the gradient-meterwill have the same aspect and the same phase as the perturbation, andthis in a ratio of constant amplitude (there is true similarity, inaspect and position, between these variations and the perturbations).

As will be explained hereinafter, this makes it possible to very exactlycompensate,by the method and/or the apparatus according to thisinvention, for the effects due to the aircraft without distortion, ordivision by some factor (such as the factor (ll/k) in the aforesaid U.S.Pat. No. 2,891,216), of the signal representing the intensity of theexternal magnetic field to be measured.

It should be noted that a precise determination of the saidbarycenter(s) and self-compensation for the stray magnetic fieldsrequires that the magnetic gradient be measured with very greataccuracy, for a perturbation of one gamma l y) on the probe or measuringhead will produce a field differential of 0.2 'y at a distance of 1.50meters. This implies achieving accuracy in measurement to the order of0.01 'y and a constant difference (i.e., the same value for the magneticgradient) irrespective of the aircraft heading. Now the magneticgradientmeter according to the aforesaid French Pat. No. 1,485,556, theprinciple of which will now be recalled, makes it possible to measurevariations in magnetic field intensity of the order of 0.001 'y, or 0.01uG, between two points. Once recorded, these variations reveal thegeneral aspect of the magnetic perturbations. v FIG. I is a highlydiagrammatic illustration of the structure of a magnetic gradient-meteraccording to the said French Pat. No. 1,485,556. In its current version,this gradient-meter comprises two probes or heads L, and L spaced by1.50 m. (D=l.50 m.) and enclosed in a boom M made of rigid plastic (inorder to maintain the two heads L, and L in a fixed mutual relationship)which is transparent" to magnetic fields.

The head or probe L,, which is preferably of the spincoupling type,functions as a nuclear oscillator, that is to say that it delivers avoltage T, at the Lannor frequency of the nuclear spins it contains,which frequency is strictly proportional to the intensity of the totalmagnetic field sensed at the point N, by probe L,. This voltage T, isamplified in an amplifier P, and its frequency is generally measured ina frequencymeter O which delivers a voltage R proportional to saidfrequency. This voltage T is filtered through a set of filters R beforebeing recorded on channel I ofa recorder V.

The second head or probe L is a nuclear filter which receives thevoltage T,, amplified in amplifier P,, at the angular frequency w,=*yH,,where 7 is the gyromagnetic ratio of the nuclear spins and H, theintensity of the total magnetic field at N,. Nuclear filter L is abandpass filter centered on the angular frequency w ='yH where H, is theintensity of the total magnetic field at N it being assumed that thenuclear filter has the same nuclear spins (of gyromagnetic ratio 7 asthe nuclear oscillator L, forming the first head. The amplitudewiseresponse curve, namely the voltage T issuing from L as a function of (0is given by the upper curve in FIG. 2.

The nuclear filter also has the effect of phase-shifting T with respectto T, when w, differs from 0),, as shown by the lower curve in FIG. 2which plots the phase-shift dd) between T and T, against 10 In fact,when H,=H w,=w and the phase shift dd) introduced by the nuclear filterL, is null, but as soon as a magnetic field gradient dH appears betweenthe points N, and N then a), differs from w, and a phase-shift dd) isintroduced by the filter. The change in phase is very rapid, being equalto 1r/2 (dd) changes from -1r/4 to +1r/4) for a change in dI-I of 5 y,thus endowing the gradient-meter with a very great sensitivity.

The voltage T issuing from nuclear filter L is amplified in an amplifierP and the difference in phase between the voltages T, and T amplified inamplifiers P, and P is determined in a phasemeter X (having two inputsX, and X which delivers (on its output X;,) a voltage T, proportional to114), and

this voltage T, is recorded on channel II of recorder V in parallel withthe voltage T which is recorded on the same recorders channel l afterbeing filtered by the filters R.

FIG. 3 recalls the structure and theory of operation of the phasemeter Xof FIG. I. This phasemeter has two channels Y, and Y comprising inseries:

in the case of the former, a phase-inverter q, a shaping unit h, (of theSchmitt-trigger type) and a differentiating and rectifying unit j,having a capacitor m,, diodes n, and p, and a resistor r,; and,

in the case of the latter, a shaping unit h; (of the Schmitt triggertype) and a differentiating and rectifying unit j having a capacitor m2,diodes n, and p and a resistor r,.

The two channels Y, and Y, drive the two inputs (for respectivelyswitching from the first to the second stage and from the second to thefirst stage) of a flip-flop of the Eccles- Jordan type, one output ofwhich drives an integrator v the output of which constitutes the outputX of phase-meter X.

Phase-meter X functions in the following manner:

Assuming first that dd: is zero, then the two voltagesT, (deduced from Tby amplification in P, and phase inversion in g) and T (deduced from T,by amplification in P,) will be in exact phase opposition, as shown;this also applies in the case of the pulsesTfiar df f shaped in h, andh, respectively. The positive pulses T f corresponding to the frontedges of zf then position themselves exactly between the positive pulsesT corresponding to the front edges of Ti as a result, the flipflop sremains for equal periods in each state and therefore delivers a signalT having equal half-waves, and the integrator v delivers null voltage.

Conversely, as soon as d is no longer equal to zero, the pulses T2} nolonger position themselves exactly between two pulses Tfi, so that thehalf-waves of T becorneasy mmetrical mid the integrator v delivers a'fisii'iie or negative voltage (depending on the source of thephase-shift d and hence of the magnetic gradient) which is not zero andwhich is delivered on X and recorded on channel II of recorder V. By wayof example, it has been possible to obtain an integrated voltage T, of:5 volts for a phase-shift of in/4, itself obtained for a gradient of i5Since the electronic noise in the unit is under one millivolt, it wastherefore possible to sense to within 0.001 y.

Lastly, the recorder V records by virtue of the precision of phase-meterX and that of frequency-meter O, which could be of the kind described inthe applicants first Certificate of Addition No. 88,663 granted on Jan.30, 1967 (appended to the applicant's French Pat. No. 1,430,874 grantedon Dec. 31, l966)simultaneously, side by side, the gradient of themagnetic field (in point of fact, the tail gradient of the aircraft) andthe absolute value of the total magnetic field.

FIG. 1 as a whole consequently furnishes, in respect of the magneticperturbations due to the aircraft, AH, (the magnetic perturbation at N,)and (AH,-AH,) which is the difference between AH the magneticperturbation at N,, and AH,, this being effected with great precision.

It will be shown by reference to FIG. 4 that it is possible to deduce,with a good degree of precision and by means of simple circuitry, thedistance x separating the barycenter (of the ferromagnetic masses) fromprobe L for instance. By ascertaining this difference along each of themain headings, it is possible to assess the positional stability of thebarycenter and to define the self-compensation possibilities for eachtype of aircraft or other carrier vehicle.

On FIG. 4 are to be seen the two heads L, and L, of FIG. 1, the head L,being closer to the source of perturbations S (the barycenter of themagnetic perturbations) producing a magnetic perturbation of intensityAH, at N where the head L, is located, and a magnetic perturbation ofintensity AH, at N,,, where the head L is located at a distance of Ax(actually 1.50 meters) from L,.

The exemplary compensation system of FIG. 4 includes: a source ofcompensating voltage V", a voltage distributor or divider Z having aslide Z, which distributes the voltage V between the partial resistorsof value R, and R and two coils W, and W, crossed respectively bycurrents I, and I, proportional to R, and R, respectively, said coils W,and W respectively generating compensating magnetic fields AH, and AH,proportional to I, and I, respectively.

For a complete compensation, we have:

The; requirement AHJAH p is met when the following three requirementsare also met:

Theheads L, and L, must have a high degree otnceumey of the order of0.0l y at least, irrespective of the position of the magnetometer headsin relation to the direction ofthe field;

The distance Ax must be very exactly known and small in comparison withx (x/Ax l0);

Adjustment of the currents I, and I, must ensure compensation.

When these three requirements are met, x can be calculated from Ax,which is known, and from p, which is determined by the divider Z. Fortaking m as the perturbing mass at S", we have:

and

. A Taking X we have:'

Ax Whence x W Thus it is indeed possible to compute x from Ax and p.

In practice, adjustment of the currents I, and 1 in the compensationcoils W, and W is effected during specific motions of the aircraft oneach heading (e.g., a rolling motion with an amplitude of and a periodof 6 seconds). I I and V are operated upon so as to simultaneouslyobtain AH,AH"=0 and (AH AH )-(AH,AH,)=O, whereupon the ratio of theresistances R /R, (which can be read off on divider Z, which divider maybe a ten-turn graduated potentiometer) gives l /l,, namely p.

Measurement of the barycenter distance x, described precedingly withreference to FIGS. 1 through 4, will enable the magnetic perturbationsdue to the aircraft to be compensated for in accordance with the methodand apparatus of the aforesaid French Pat. No. 1,485,557: it willsuffice to apply to the compensation coils 16, 17 of that patent thevoltage representing the difference (AH AH,). However, the greataccuracy with which the method hereinbefore described provides ameasurement of the magnetic field gradient in the region of sensor L,authorizes direct electronic compensation by subtracting, from thevoltage T, which is delivered by frequency-meter. Q and is proportionalto the intensity of the total magnetic field (H,+AH,) at N,, a voltageproportional to AH, that is reconstituted by applying the principalteaching of the invention, as will be set forth hereinbelow in detailwith reference to FIG. 5.

The explanations relevant to the preliminary stages in the study of theperturbing fields being thus complete, there will now be described howthe teachings of this invention can be applied to compensaterespectively for the stray fields due to the ferromagnetic masses of theaircraft (FIG. 5) and the eddy currents (FIGS. 6 and 7).

The great sensitivity of the gradient measurement makes it possible tomove the two heads (nuclear oscillator and nuclear filter) of thesubject device of this invention, which correspond to the heads L, andL, of FIG. 4, closer to within a small distance of each other, I meterfor instance.

1t is then possible to write directly the classic equation:

d(AH1) due to the fact that H varies as llx (the symbol d/dxrepresenting the derivative with respect to x). When Ax is small, thedifference (AH AH,) substantially represents the aircraft at thelocation of the head or probe L All that is therefore necessary is tomultiply (or AH AHJ by a constant factor x/3 and to subtract this valuefrom the measurement taken by the head L to cancel out the perturbationsproduced by these magnetic masses at N,, since 11",. which in to becompensated for. is equal precisely to dorm A device for performing theabove operations is illustrated in FIG. 5. It comprises in combination:

a magnetometer head 3 (consisting for instance of a nuclear oscillatorof the kind described in the aforesaid French Pat. No. 1,485,556)located at the first point 1 at a first distance d (d,=x+Ax) from thebarycenter 4 of the perturbing magnetic fields (with the arrow 3representing the perturbing magnetic fields) and capable of generating afirst e.m.f.f of frequency proportional to the intensity (H-l-AHQ of thetotal magnetic field (H, being the external magnetic field to bemeasured and AH, the perturbing field at the point 1) at said firstpoint 1;

a frequency-meter 5 (for instance of the type described in the aforesaidFrench Certificate of Addition No. 88,663) connected to the output 6 ofsaid magnetometer head 1 l preferably via an amplifier 7whereby todeduce from said first e.m.f. f, a first voltage e, proportional to saidfrequency and hence to said intensity (H,,+AH,) of the total magneticfield;

a nuclear filter 8 (preferably of the type described in the aforesaidFrench Pat. No. 1,485,556) positioned at a second point 2 at a seconddistance d (d x) from said barycenter 4 and having its input 9 connectedto the output 6 of said head lpreferably via the amplifier 7-whereby toreceive therefrom said first e.m.f. f, and deliver on its output 10 asecond e.m.f. f which, with respect to said first e.m.f. f presents aphase difference proportional to the difference (Am-AH between theintensities (H +AH and (H,,+AH,) of the total magnetic field at thesecond point 2 and the first point 1 (AH being the perturbing field atthe point 2);

a phasemeter 11 (for instance of the type described in the aforesaidFrench Pat. No. 1,485,556 and illustrated in FIG. 3

of the drawings accompanying the present description) having two inputs12, 13 connected respectively to the output 6 of said magnetometer headl-preferably via the amplifier 7 (whereby to receive therefrom saidfirst e.m.f.f1) and to the output 10 of nuclear filter 8preferably viaan amplifier 14- whereby to receive therefrom said second e.m.f. f andan output 15 which delivers a second voltage e2 proportional to saiddifference (Alb-AH between the intensities of the total magnetic field;

an adjustable-gain amplifier 16 the input 17 of which is connected tothe output 15 of said phasemeter 11 whereby to receive said secondvoltage a, and deduce therefrom a third voltage a subtraction unit 18having two inputs 19, 20 connected respectively to the output 21 of saidfrequency-meter 5 whereby to receive therefrom said first voltage e, andto the output 22 of said amplifier 16 whereby to receive therefrom saidthird voltage 2 and an output 24 delivering a fourth voltage e,proportional to the difference between said first voltage e applied toits first input 19 and said third voltage e applied to its second input20; and

means for measuring said fourth voltage e, which is proportional to theintensity H, of the external magnetic field to be measured at said firstpoint 1, assuming proper adjustment of the gain of amplifier 16 anddeterminate positions of said magnetometer head 3 and said nuclearfilter 8, to wit substantially on the fore-aft axis 25 of the aircraftor other carrier vehicle wherever this is possible, said means includingfor instance a filter set 26 for filtering the desired components of e,,followed by a recorder 27.

The above-described system functions in the following manner:

The nuclear oscillator or magnetometer head 3 delivers an e.m.f.f, offrequencyf proportional to (H,,+AH,). This e.m.f. f amplified inamplifier 7, is delivered:

to the frequency-meter 5 which delivers a voltage e, of am plitude a,=k,(HA-AH. where k, is a constant;

to nuclear filter 8 which delivers an e.m.l'.f the phase-shift dd: ofwhich with respect to the c.m.f.f delivered by oscillator 3 isproportional to (AH AH,

to phasemeter 11 which also receives the e.m.f.f delivered by filter 8and amplified in amplifier l4 and delivers a voltage e of amplitude a=k,(AH AH,), where k is a constant which may be equal to k,. The voltagee; is multiplied in amplifier 16 by a factor 4) dependent upon thedistance d,=x, so as to obtain a voltage e of amplitude a =(x)k AHrAH,The value (x) is chosen so that the requirement k /k,=x/3 is met, forthen a k,x/3(AH AH,

It is to be noted that if k =k,, then d =x/3.

The subtraction unit 18 thus receives a voltage e, of amplitudea,=k,(H,,+AH on its input 19 and a voltage a of amplitude x 11 (AH (asexplained hereinbefore with reference to P10. 4) on its input 20 andconsequently delivers on its output 24 a voltage a, amplitude a =k (H+AH,)k,AH,=k,H,,. This shows that the amplitude 0 of the voltage 2 isstrictly proportional to the field H,, to be measured. It is to be notedthat the coefficient k is by no means a fraction factor like the factorll/k) of the aforesaid U.S. Pat. No. 2,89l,2l6, which reduces accuracy,but merely a proportionality factor resulting from the amplifier 7 andthe frequency-meter 5, which start with an e.m.f.f, not divided by areduction factor.

The voltage e represents the intensity of the earth's magnetic field H,(to which it is strictly proportional) and its perturbations, regardlessof the motions described by the carrier aircraft or the magnetic massesthereof (but with the proviso to be made hereinbelow in respect of eddycurrents).

The bandpass filter or set thereof 26 allows the recorder 27 to recordanomalies with frequencies corresponding to those of the sought afteranomalies and permits maximum possible elimination of the naturalperturbations and gradients (horizontal and vertical) of the earth'smagnetic field.

As it has just been described, the system for compensating for themagnetic effects due to the aircraft or other carrier vehicle isnonetheless incomplete since the motion of the aircraft through theearth's magnetic field generates eddy currents in the conductingsurfaces of the aircraft, notably the fuselage, and these eddy currentsdistribute stray magnetic fields which are all the greater as the flightspeed of the aircraft is higher.

Fortunately, the structural symmetry of the aircraft or carrier vehicleenables these phenomena to be compensated for in the same way aspermanent or induced parasitic magnetic fields. Only the location of thebarycenter of the magnetic forces due to eddy currents differs from thatof the barycenter of permanent or induced magnetic fields. Therefore, inorder to make a single overall compensation setting possible (asindicated precedingly), it is necessary to make these two barycenterscoincide and to consequently alter the aspect of the magnetic fieldgradient produced by the eddy currents. An explanation will now be givenof how this can be accomplished (see FIGS. 6 and 7).

In accordance with a second teaching of this invention, the process forcompensating for magnetic perturbations engendered by eddy currents inthe aircraft or other vehicle (having its axis as at in FIG. 6) includesthe steps of generatingnotably by means of coils 28 energizable with thevoltage picked off a set of generating coils 29 which have mutuallyorthogonal axes and are rigidly connected to the aircraft, said voltagebeing adjustable for amplitude by an adjustable resistor adjacent saidsecond point 2 (at which filter 8 is located, oscillator 3 being locatedat the point 1), a corrective magnetic field Ah in phase opposition withthe perturbing field Ah resulting from the barycenter of the eddycurrents, the value of this correctivefield Ah, being set so as to causesaid barycenter of the magnetic perturbations produced by the.

eddy currents to coincide with the barycenter of the ferromagneticmasses (the barycenter of the eddy currents being shifted from its realposition Br to its virtual position Bv by the corrective field Ah Moreprecisely, consideration may be given to the case of the magnetic fieldsgenerated by the motions of the fuselage during North-South andSouth-North pitching motions. The real barycenter Br produces on thehead 3 and the filter 8 field variations which are Ah, and Ahrespectively. The coils 28 generate a field Ah (in phase opposition withAh The resulting gradient (Ah --Ah )Ah,, for a constant Ah,, gives avirtual barycenter in relation to the head 3 that appears to be all thefarther as (Ah -Ah is closer to M1,: the barycenter moves towardsinfinity as (Ah 'Ahfl tends towards Ah, (FIG. 7 By using adjustableresistor 30 to set the current intensity through the coils 28 and hencethe intensity of the field Ah it is thus possible to shift theeddy-current barycenter to make it coincide with the barycenter of theferromagnetic masses (the barycenter of the eddy currents moves from itsreal position Br to its virtual position bv which is the barycenter ofthe ferromagnetic masses). In FIG. 7, the curves 31 and 32 represent thegradients due to the virtual barycenter and the real barycenter of theeddy currents respectively.

This possibility stems from the formula established precedingly,

Ah Ah with p= Ah If, with Ah, constant, Ah is caused to vary, then pwill vary, resulting in an apparent variation of x (in addition to thevalue x, FIG. 7 bears the value x corresponding to the real barycenterBr).

Since the measurements of the distances of the barycenters of the eddycurrents and of the ferromagnetic masses reveal, in the case of the eddycurrents, a resultant perturbation closer to the points 1 and 2 than inthe case of the ferromagnetic masses, Ah must be negative, which, whenthe resistor 30 is adjusted, causes an apparent receding of theeddy-current barycenter, which shifts from Br to Bv, as shown.

It should be noted that, instead of being energized by generating coilssubjected to the eddy-current-generating magnetic field, the coils 28can if necessary be energized by any other means capable of producing acurrent similar to these eddy currents, such as a minicomputer forinstance,

Lastly, FIG. 8 illustrates a second form of embodiment of a compensatedmagnetometer according to the invention, the circuitry in FIG. 8 being avariant on that shown in FIG. 5 (in point of fact, FIG. 8 differs fromFIG. 5 only in respect of the portion boxed in with broken lines in FIG.8).

The compensated magnetometer in FIG. 8 comprises, in combination:

a first magnetometer head of the same type as the magnetometer head 3 inFIG. 5 and located at a first point 1 at a distance d, from thebarycenter 4 of the perturbing magnetic fields, this head being capableof generating, on its output, a first e.m.f. f, at a frequencyproportional to the intensity (H,,+ Ah,) of the total magnetic field atthat point 1;

a frequency-meter 5 of the same type as the frequencymeter 5 in FIG. 5,connected to the output 6 of magnetometer head l.-preferably via anamplifier 7-and deducing from the e.m.f.f, a first voltage 2,proportional to the frequency of the e.m.f.f, and hence to (H,,+Ah,);

a second magnetometer head 30 identical to magnetometer head 3, locatedat a second point 2 at a second distance d, (d =x) .from the barycenter4 and capable of generating a second e.m.f. f,, at a frequencyproportional to the intensity (H d-Ah,) of the total magnetic field atthat second point 2;

a differential frequency-meter 31 (for instance of the type described inthe patent application filed by the Applicant in this country on Oct.14, 1969 for differential frequency-me ter") having two inputs 32, 33connected respectively to the output 6 of said magnetometer head 1,preferably via amplifier 7, whereby to receive the first e.m.f. f,therefrom, and to the output 6a of the second magnetometer head 30,preferably via an amplifier 7a, whereby to receive the second e.m.f. ftherefrom, and an output 35 which delivers a second voltage eproportional to the difference (AH AH,) between the intensities of thetotal magnetic field at the points 2 and 1;

an adjustable-gain amplifier 16 having its input 17 connected to theoutput 35 of said differential frequency-meter 31 whereby to receive thesecond voltage a, and deduce therefrom a third voltage e;,;

a subtraction unit 18 having two inputs 19, 20 respectively connected tothe output 21 of frequency-meter 5 and to the output 22 of amplifier 16,whereby to receive e, and e, therefrom respectively, this unit 18delivering from its output 24 a fourth voltage e, proportional to thedifference between e, and e and means for measuring said fourth voltagee, which is proportional'to the intensity H, of the external magneticfield to be measured at said point 1, assuming proper adjustment of thegain in'amplifier 16 and determinate positions of said magnetometerheads 3 and 3a, said means including for instance a filter set 26followed by a recorder 27.

The magnetometer in figure 8 functions in the following manner:

The nuclear oscillators or magnetometer heads 3 and 3a respectivelydeliver an e.m.f. f, of frequency proportional to (H,,+A-H,) and ane.m.f. f, of frequency proportional to (H -l- AH These e.m.f.f, ande.m.f.f,, are amplified in amplifiers 7 and 7a respectively.

The differential frequency-meter 31 determines the difference betweenthe frequencies of the e.m.f. f, and e.m.f. f and delivers from itsoutput 35 a voltage e, of amplitude a; k2(AH -AH where k is a constantwhich may be equal to k, while frequency-meter 5 measures the frequencyof the e.m.f. f, andconsequently delivers a voltage e, of amplitude a,==

k,(Hnl AH, where k, is a constant.

Iii the embodiment shown in figure 8, the voltage e is multiplied inamplifier 16 by a factor 4) dependent on the distance d,=x, so as toobtain a voltage 2 of amplitude a y(x)k (AH AH,). (x) is chosen so thatthe requirement k /k,=x/ 3 is met, for then a =k,x/3(AH,AH,).

It is to be noted that if k =k,, then =x/3.

Hence, as with the embodiment of FIG. 5, the subtraction unit 18receives a voltage e, of amplitude a,=k,(H,,+AH,) on its input 19 and avoltage e; of amplitude The circuitry of FIG. 8 offers the sameadvantages as that of FIG. 5, but with the additional possibility ofoperating in real time with a faster response since it is devoid of anuclear filter (in which the spins are subjected to the magnetic fieldat the point 2 and which receives, on its input, an e.m.f. of frequencyproportional to the magnetic field at point l).

A compensated magnetometer devised in accordance with either of theprecedingly described embodiments offers numerous advantages bycomparison with prior art magnetometers, the following beingoutstanding:

The degree of accuracy obtained is very high.

All parasitic or stray magnetic fields are automatically compensatedfor.

The compensation process produces neither harmful magnetic fields in theregion where the instruments aboard the aircraft or other carriervehicle are located (as in the case of the aforesaid French Pat. No.1,485,557) nor a reduction in the useful signal proportional to theintensity of the magnetic field to be measured (as in the case of theaforesaid US. Pat. No. 2,891,216).

it goes without saying that many changes may be made to the forms ofembodiment described hereinabove without departing from the scope of theinvention and that, broadly speaking, the invention is by no meanslimited to the exemplary applications considered herein.

What is claimed is: 1. A method of compensating for perturbing magneticfields in a carrier vehicle in order to permit the accurate measurementof the intensity of an external magnetic field and its perturbations,comprising the steps of:

generating a first voltage proportional, with a first proportionalityfactor k,, to the intensity of the total magnetic field at a firstpoint, located in the carrier vehicle, at a first distance x from thebarycenter of the perturbing magnetic fields; generating a secondvoltage proportional, with a second proportionality factor k to thedifference between the intensities of the total magnetic field at saidfirst point and at a second point, located in the carrier vehicle, at asecond distance from said barycenter; multiplying said second voltage bya factor (it dependent on said first distance to obtain a third voltageproportional, with said first proportionality factor, to the intensityof the perturbing magnetic field at said first point;

subtracting said third voltage from said first voltage to obtain afourth voltage; and,

determining the amplitude of said fourth voltage, which is proportionalto the intensity of the external magnetic field to be measured at saidfirst point, rid of the influence of the perturbing magnetic fields;

the first distance x, the distance Ax between said first and secondpoints, the first proportionality factor k,, the second proportionalityfactor k said factor (ii, and the ratio p between the intensities of theperturbing magnetic fields at said second point and said first pointrespectively being related by the formulas Ax=x 1) and, k -lk ,=x/3.

2. A method according to claim 1 including the additional step ofgenerating, in order to compensate for the proportion of the perturbingmagnetic fields engendered by eddy currents in the carrier vehicle, acorrective magnetic field adjacent said second point and proportional tosaid proportion of the perturbing magnetic fields engendered by eddycurrents in the carrier vehicle, the amplitude of said correctivemagnetic field being set so as to cause the barycenter of the proportionof the perturbing magnetic fields engendered by said eddy currents tocoincide with the barycenter of the perturbing magnetic fields caused bythe ferromagnetic mass of the carrier vehicle.

3. A method according to claim 1 wherein the steps of determining theratio p include:

disposing two coils at said first and second points;

energizing each coil from a variable source of current;

causing the carrier vehicle to describe predetermined motions inpredetermined directions;

measuring the magnetic field at the first point and the magnetic fieldgradient between the first and second points; and,

adjusting the amplitudes of each of the variable currents applied tosaid coils until, simultaneously, said magnetic field remains constantand said magnetic field gradient remains null during said motions.

4. Apparatus for compensating for perturbing magnetic fields in acarrier vehicle in order to permit the accurate measurement of theintensity of an external magnetic field and its perturbations,comprising, in combination:

a first magnetometer head located at a first point in said carriervehicle, substantially on the longitudinal axis thereof, at a firstdistance from the barycenter of the perturbing magnetic fields andcapable of generating, on its output, an e.m.f. having a frequencyproportional to the intensity of the total magnetic field at said firstpoint;

a frequency-meter connected to the output of said first magnetometerhead to receive said e.m.f. therefrom and generate a first voltageproportional to said frequency, and, hence, to said total magnetic fieldintensity;

a second magnetometer head located at a second point in said carriervehicle, substantially on the longitudinal axis thereof, at a seconddistance from said barycenter and forming with said first head amagnetic gradient-meter capable of delivering a second voltageproportional to the difference between the intensities of the totalmagnetic field at said second and first point;

a variable-gain amplifier the input of which is connected to the outputof said gradient-meter to receive said second voltage and generate athird voltage, the gain of said variable-gain amplifier being adjustedin a predetermined manner;

a subtraction unit having two inputs connected respectively to theoutput of said frequency-meter to receive said first voltage therefromand to the output of said amplifier to receive said third voltagetherefrom, and an output delivering a fourth voltage proportional to thedifference between said first voltage applied to its first input andsaid third voltage applied to its second input; and,

means for measuring said fourth voltage which is proportional to theintensity of the external magnetic field to be measured at said firstpoint.

5. Apparatus according to claim 4 wherein said second magnetometer headcomprises:

a nuclear filter located at said second point, said nuclear filterhaving its input connected to the output of said first magnetometer headso as to receive therefrom said e.m.f. constituting a first e.m.f. andto deliver, on its output, a second e.m.f. exhibiting with respect tosaid first e.m.f. a phase-difference proportional to the differencebetween the intensities of the total magnetic field at said second pointand said first point; and,

a phase-meter having two inputs connected respectively to the output ofsaid first magnetometer head to receive said first e.m.f. therefrom andto the output of said nuclear filter to receive said second e.m.f.therefrom, and an output which delivers said second voltage.

6. Apparatus according to claim 5 including, for the purpose ofcompensating for the perturbing magnetic fields set up by the eddycurrents engendered by the carrier vehicle, at least one coil disposedadjacent said second point and a source ofelectric current of adjustableamplitude connected for energizing said coil, the current passingthrough said coil being such that the coil generates, adjacent saidsecond point, a corrective magnetic field in phase opposition with theperturbing magnetic fields set up by said eddy currents, at an intensitysuch that it causes the barycenter of said perturbing magnetic fieldsset up by said eddy currents to coincide with the barycenter of theperturbing magnetic fields caused by the ferromagnetic mass of thecarrier vehicle.

7. Apparatus according to claim 6 wherein said source is formed by atleast one generating coil mounted within the carrier vehicle.

8. Apparatus according to claim 7 wherein said source is formed by threegenerating coils having mutually perpendicular axes.

9. Apparatus according to claim 4 wherein said second magnetometer headis capable, like said first head, of generating on its output a seconde.m.f. at a frequency proportional to the intensity of the magneticfield at said second point; and, including a differentialfrequency-meter having two inputs connected respectively to the outputof said first magnetometer head to receive said first e.m.f. therefromand to the output of said second magnetometer head to receive saidsecond e.m.f. therefrom, and an output which delivers said secondvoltage.

10. Apparatus according to claim 9 including for the purpose ofcompensating for the perturbing magnetic fields set up by the eddycurrents engendered by the carrier vehicle, at least one coil disposedadjacent said second point and a source of electric current ofadjustable amplitude connected for energizing said coil, the currentpassing through said coil being such that the coil generates, adjacentsaid second point, a corrective magnetic field in phase opposition withthe perturbing magnetic fields set up by said eddy currents, at anintensity such that it causes the barycenter of said perturbing magneticfields set up by said eddy currents to coincide with the barycenter ofthe perturbing magnetic fields caused by the ferromagnetic mass of thecarrier vehicle.

11. Apparatus according to claim 10 wherein said source is formed by atleast one generating coil mounted within the carrier vehicle.

12. Apparatus according to claim 11 wherein said source is formed bythree generating coils having mutually perpendicular axes.

13. Apparatus according to claim 4 including for the purpose ofcompensating for the perturbing magnetic fields set up by the eddycurrents energized by the carrier vehicle, at least one coil disposedadjacent said second point and a source of electric current ofadjustable amplitude connected for energizing said coil, the currentpassing through said coil being such that the coil generates, adjacentsaid second point, a corrective magnetic field in phase opposition withthe perturbing magnetic fields set up by said eddy currents, at anintensity such that it causes the barycenter of said perturbing magneticfields set up by said eddy currents to coincide with the barycenter ofthe perturbing magnetic fields caused by the ferromagnetic mass of thecarrier vehicle.

14. Apparatus according to claim 13 wherein said source is formed by atleast one generating coil mounted within the carrier vehicle.

15. Apparatus according to claim 14 wherein said source is formed bythree generating coils having mutually perpendicular axes.

1. A method of compensating for perturbing magnetic fields in a carriervehicle in order to permit the accurate measurement of the intensity ofan external magnetic field and its perturbations, comprising the stepsof: generating a first voltage proportional, with a firstproportionality factor k1, to the intensity of the total magnetic fieldat a first point, located in the carrier vehicle, at a first distance xfrom the barycenter of the perturbing magnetic fields; generating asecond voltage proportional, with a second proportionality factor k2, tothe difference between the intensities of the total magnetic field atsaid first point and at a second point, located in the carrier vehicle,at a second distance from said barycenter; multiplying said secondvoltage by a factor phi dependent on said first distance to obtain athird voltage proportional, with said first proportionality factor, tothe intensity of the perturbing magnetic field at said first point;subtracting said third voltage from said first voltage to obtain afourth voltage; and, determining the amplitude of said fourth voltage,which is proportional to the intensity of the external magnetic field tobe measured at said first point, rid of the influence of the perturbingmagnetic fields; the first distance x, the distance Delta x between saidfirst and second points, the first proportionality factor k1, the secondproportionality factor k2, said factor phi , and the ratio Rho betweenthe intensities of the perturbing magnetic fields at said second pointand said first point respectively being related by the formulas Delta xx( Rho -1) and k2 . phi /k1 x/3.
 2. A method according to claim 1including the additional step of generating, in order to compensate forthe proportion of the perturbing magnetic fields engendered by eddycurrents in the carrier vehicle, a corrective magnetic field adjacentsaid second point and proportional to said proportion of the perturbingmagnetic fields engendered by eddy currEnts in the carrier vehicle, theamplitude of said corrective magnetic field being set so as to cause thebarycenter of the proportion of the perturbing magnetic fieldsengendered by said eddy currents to coincide with the barycenter of theperturbing magnetic fields caused by the ferromagnetic mass of thecarrier vehicle.
 3. A method according to claim 1 wherein the steps ofdetermining the ratio Rho include: disposing two coils at said first andsecond points; energizing each coil from a variable source of current;causing the carrier vehicle to describe predetermined motions inpredetermined directions; measuring the magnetic field at the firstpoint and the magnetic field gradient between the first and secondpoints; and, adjusting the amplitudes of each of the variable currentsapplied to said coils until, simultaneously, said magnetic field remainsconstant and said magnetic field gradient remains null during saidmotions.
 4. Apparatus for compensating for perturbing magnetic fields ina carrier vehicle in order to permit the accurate measurement of theintensity of an external magnetic field and its perturbations,comprising, in combination: a first magnetometer head located at a firstpoint in said carrier vehicle, substantially on the longitudinal axisthereof, at a first distance from the barycenter of the perturbingmagnetic fields and capable of generating, on its output, an e.m.f.having a frequency proportional to the intensity of the total magneticfield at said first point; a frequency-meter connected to the output ofsaid first magnetometer head to receive said e.m.f. therefrom andgenerate a first voltage proportional to said frequency, and, hence, tosaid total magnetic field intensity; a second magnetometer head locatedat a second point in said carrier vehicle, substantially on thelongitudinal axis thereof, at a second distance from said barycenter andforming with said first head a magnetic gradient-meter capable ofdelivering a second voltage proportional to the difference between theintensities of the total magnetic field at said second and first point;a variable-gain amplifier the input of which is connected to the outputof said gradient-meter to receive said second voltage and generate athird voltage, the gain of said variable-gain amplifier being adjustedin a predetermined manner; a subtraction unit having two inputsconnected respectively to the output of said frequency-meter to receivesaid first voltage therefrom and to the output of said amplifier toreceive said third voltage therefrom, and an output delivering a fourthvoltage proportional to the difference between said first voltageapplied to its first input and said third voltage applied to its secondinput; and, means for measuring said fourth voltage which isproportional to the intensity of the external magnetic field to bemeasured at said first point.
 5. Apparatus according to claim 4 whereinsaid second magnetometer head comprises: a nuclear filter located atsaid second point, said nuclear filter having its input connected to theoutput of said first magnetometer head so as to receive therefrom saide.m.f. constituting a first e.m.f. and to deliver, on its output, asecond e.m.f. exhibiting with respect to said first e.m.f. aphase-difference proportional to the difference between the intensitiesof the total magnetic field at said second point and said first point;and, a phase-meter having two inputs connected respectively to theoutput of said first magnetometer head to receive said first e.m.f.therefrom and to the output of said nuclear filter to receive saidsecond e.m.f. therefrom, and an output which delivers said secondvoltage.
 6. Apparatus according to claim 5 including, for the purpose ofcompensating for the perturbing magnetic fields set up by the eddycurrents engendered by the carrier vehicle, at least one coil disposedadjacent said second point and a source of electric current ofAdjustable amplitude connected for energizing said coil, the currentpassing through said coil being such that the coil generates, adjacentsaid second point, a corrective magnetic field in phase opposition withthe perturbing magnetic fields set up by said eddy currents, at anintensity such that it causes the barycenter of said perturbing magneticfields set up by said eddy currents to coincide with the barycenter ofthe perturbing magnetic fields caused by the ferromagnetic mass of thecarrier vehicle.
 7. Apparatus according to claim 6 wherein said sourceis formed by at least one generating coil mounted within the carriervehicle.
 8. Apparatus according to claim 7 wherein said source is formedby three generating coils having mutually perpendicular axes. 9.Apparatus according to claim 4 wherein said second magnetometer head iscapable, like said first head, of generating on its output a seconde.m.f. at a frequency proportional to the intensity of the magneticfield at said second point; and, including a differentialfrequency-meter having two inputs connected respectively to the outputof said first magnetometer head to receive said first e.m.f. therefromand to the output of said second magnetometer head to receive saidsecond e.m.f. therefrom, and an output which delivers said secondvoltage.
 10. Apparatus according to claim 9 including for the purpose ofcompensating for the perturbing magnetic fields set up by the eddycurrents engendered by the carrier vehicle, at least one coil disposedadjacent said second point and a source of electric current ofadjustable amplitude connected for energizing said coil, the currentpassing through said coil being such that the coil generates, adjacentsaid second point, a corrective magnetic field in phase opposition withthe perturbing magnetic fields set up by said eddy currents, at anintensity such that it causes the barycenter of said perturbing magneticfields set up by said eddy currents to coincide with the barycenter ofthe perturbing magnetic fields caused by the ferromagnetic mass of thecarrier vehicle.
 11. Apparatus according to claim 10 wherein said sourceis formed by at least one generating coil mounted within the carriervehicle.
 12. Apparatus according to claim 11 wherein said source isformed by three generating coils having mutually perpendicular axes. 13.Apparatus according to claim 4 including for the purpose of compensatingfor the perturbing magnetic fields set up by the eddy currents energizedby the carrier vehicle, at least one coil disposed adjacent said secondpoint and a source of electric current of adjustable amplitude connectedfor energizing said coil, the current passing through said coil beingsuch that the coil generates, adjacent said second point, a correctivemagnetic field in phase opposition with the perturbing magnetic fieldsset up by said eddy currents, at an intensity such that it causes thebarycenter of said perturbing magnetic fields set up by said eddycurrents to coincide with the barycenter of the perturbing magneticfields caused by the ferromagnetic mass of the carrier vehicle. 14.Apparatus according to claim 13 wherein said source is formed by atleast one generating coil mounted within the carrier vehicle. 15.Apparatus according to claim 14 wherein said source is formed by threegenerating coils having mutually perpendicular axes.